1. Field of the Invention
The present invention relates generally to a viscous fluid drive device. In particular, the present invention is concerned with a ramped clutch plate of a viscous fluid fan clutch for a vehicle.
2. Description of the Related Art
A thermostatically-controlled viscous fluid clutch assembly for driving and rotating a vehicle cooling fan is well-known. A multi-bladed fan is removably secured to a body of the clutch. The fan and clutch assembly are installed between an accessory pulley (typically the water pump pulley) of a vehicle engine and a radiator. The clutch drives the fan at high speeds close to input speed when cooling is required and permits the fan to rotate at low speeds when cooling is not required. Thermostatic control of the fan through the clutch reduces airflow noise caused by fan rotation and the load on an engine, resulting in horsepower gain and improved fuel economy.
A clutch plate, housed within the clutch, having lands and grooves is mated to the body having complementary lands and grooves. A pump plate divides the interior volume of the clutch into a pair of fluid chambers, a working chamber and a reservoir. Fill openings in the pump plate permit selective flow of a viscous fluid from the reservoir to the working chamber and into a shear zone between the lands and grooves of the body and clutch plate. Fluid shear at the lands and grooves transfers input torque from the clutch plate to drive the body and the attached fan.
When cooling is not required, the fill openings in the pump plate are closed and the fluid in the shear zone is pumped into a pumping chamber. Orifices in the pump plate permit passage of the fluid from the pumping chamber into the reservoir. Many conventional clutches employ wipers to enhance the pump-out of fluid from the pumping chamber. The removal of a majority of the fluid from the shear zone substantially reduces the shear between the clutch plate and the body, thereby substantially reducing the rotation of the fan.
Various types of wipers are known. Many clutches utilize separately-formed wipers which are mounted (welded) on the pump plate. While such wipers effectively increase pressure in a pumping chamber to enhance pump-out, they contribute additional material and assembly costs to the clutch. Other clutches utilize wipers stamped into the pump plate. Stamped wipers are less costly than separately-formed wipers but generally are not as efficient for pump-out.
The art continues to seek improvements. It is desirable that fluid in a clutch be effectively and efficiently pumped from a working chamber to a reservoir to disengage a clutch and reduce fan rotation upon thermostatic control.